1. Field of the Invention
The present invention relates to an image signal processing system in which an image is processed in the state of electrical image signals.
2. Description of the Prior Art
There are already proposed image processings such as transmission or storage by converting an image into electrical signals.
A binary representation of an A3-sized image with a resolving power of 400 dpi requires image signals of ca. 4M bytes, so that the storage of said image signals requires a corresponding memory capacity.
It is therefore proposed to compress the image by run length encoding at the storage, such compression is capable of reducing the amount of data of an ordinary image to ca. 1/10, thus enabling the use of a memory of a relatively limited capacity. However, image signals subjected to a dither process for pseudo continuous tone reduction are not rich in continuous same signals, so that the signal compression, for example, by run length encoding may result in a larger data amount after the compressing process, thus requiring a significantly large memory capacity.
Also the storage of plural images naturally requires a correspondingly enlarged memory capacity.
For this reason image compression is contemplated at the storage of images. Such compression allows one to reduce the amount of data of ordinary images to ca. 1/10, so that the storage of plural images can be achieved with a memory of a relatively small capacity. However, the amount of data after compression is variable according to the contents of images, so that the memory area required for storage cannot be determined in advance, and a memory area prepared in advance may not be enough for storing the data.
Also the amount of compressed data is variable according to the content of image, so that the amount of data per line is not constant. Consequently, the generated image will be blurred unless exact line synchronization is conducted at the expansion of image signals thus compressed and stored, and satisfactory image regeneration cannot be expected with such uncontrolled expansion. In the event of such expansion error, there is conducted an error processing, for example, forbidding the image output of a line of such error. Consequently, the number of lines of image stored in fact in the memory may be different from the number of output lines. Also a compression of image for example with run length encoding results in a varying amount of data after compression, according to the content of image. Consequently, the amount of data stored in the memory varies from image to image, and the amount of data to be read from the memory at the image expansion is also variable. In this manner the readout and expansion of the compressed image cannot be achieved in a uniform way.
On the other hand, in order to represent the image density in multiple levels with digital image signals, the number of bits of image signals corresponding to a pixel has to be increased. For example, two levels of black and white require a binary digital signal, while three or more density levels including intermediate densities require ternary digital signals or higher. However, the storage of an A3-sized image in two levels and with a resolving power of 400 dpi requires a memory capacity of ca. 4 M bytes, and an even high capacity is required for the storage of a larger number of density levels. Consequently, if the image signals are stored in binary signals in consideration of the memory capacity and cost, there will result a density difference between the image formed by the original image signals and the image formed from the stored image signals, if the original image signals have three or more levels.